Pressure resistant caisson

ABSTRACT

A monolithic offshore platform includes a number of vertical, cylindrical cells which are monolithically attached to each other at the contact points. The cells will therefore circumferentially be subjected to pure compressive forces. In one embodiment, all the cells have the same outer diameter. There is one central cell and around this there are first six cells with the centers on a circle concentric with the central cell, thereafter 12 cells are placed outside. If desired, new rows of cells can be added on the outside. The structure can also be constructed without the central cell. The cells can be closed with a spherical shell in each end.

United States Patent m1:

Mo Apr. 29, 1975 PRESSURE RESISTANT CAISSON OTHER PUBUCATIONS [76]Inventor: Olav Mo, Groensundveien 94. T l4 1 6 I 360Nesbru Norway he Olland Gas Journal of Sept. 970. pp. 0 l. [22] Filed: Apr. 23, 1973 PrimarEmminer-Jacob Shapiro [2 I] Appl' No: 353,538 Attorney, Agem, orFirm-Larson, Taylor and Hinds 30 F A I r P 0 D [57] ABSTRACT I l I f pp"9" am 7 A monolithic offshore platform includes a number of Ma 197;vertical, cylindrical cells which are monolithically at- DOC. 5. l97447N727 tached to each other at the Contact points. The cells willtherefore circumferentially be subjected to pure [5?] IU.S. Cl til/46.5;6l/50 compressive forces In one embodiment a" the cells 2 27/38 [.36535/00 have the same outer diameter. There is one central I e 0 T cell andaround this there are first six cells with the 5 centers on a circleconcentric with the central cell, 6] References cued thereafter l2 cellsare placed outside. If desired. new

N T STATES PATENTS rows of cells can be added on the outside. The struc-2.661.600 12/1953 Hopkins..... 6l/46.5 ture can also be constructedwithout the central cell. 3.434.442 3/!969 Manning.... 61146.5 X Thecells can be closed with a spherical shell in each 3.535.884 [0/1970Chaney (Si/46.5 end 3.708.981 H1973 Roulet 61/46 3.793.842 2/1914Lucroix 6l/46.5 8 a 13 Drawing Figures .QAMVH,

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mam; 45 -,P- I swam w IZZY/M4474 PRESSURE RESISTANT CAISSON Theinvention relates to a caisson of concrete for location on the seafloor. Such caissons are particularly useful as foundations forplatforms and as oil storage.

A concrete structure should principally not be subjected to tension.This increases cost because reinforcements are necessary to take tensileforces. Tensile forces also increase the danger of cracks withaccompanying leakage, corrosion of the reinforcing material, etc. On theother hand, a concrete construction is very well suited for takingcompressive forces because concrete per se is cheaper than steel inconnection with compressive forces and because concrete structuresbecome so coarse that breaking and local stresses rarely becomeproblems.

The object of the present invention is to find a caisson which generallyis subjected to compressive forces while having marine qualitiesnecessary for structures which are to be floated and submerged at sea.

FIG. 1 is a schematic side elevational view partially in cross sectionof a caisson showing the principle features of the invention;

FIG. 2 is a view taken along line 1-1 of FIG. 1 showing a cellconfiguration according to the invention;

FIGS. 3 through I2 are schematic diagrams showing successive stages ofconstruction of a caisson according to the invention; and

FIG. 13 is a view similar to that shown in FIG. 2 showing an alternativecell configuration.

The invention will be described in the following with reference to theexample shown on the FIGS. 1-2. The caisson consists of a number ofvertical, cylindrical cells which are monolithicly attached to eachother at the contact points. The cells will therefor circumferentiallybe subjected to pure compressive forces. Since all the cells are of thesame size, there will be no fixed moments at the contact points,deformations appearing only as changes of scale. Practically speaking,the cells will act independently which is of great importance if a wallshould collapse in an accident. If the number of cells is great enough,as for instance is shown on the drawing, the structure is functionaleven when it is collapsed locally. Looking at the individual operations,the following result is evident:

During construction (see below) collapse of one cell will only make thestructure take a certain list. The same is true during towing. on thecondition that the free-board is sufficient. If a list is present, itwill increase during submerging but, if the structure has one or moretowers as shown on the figures, the structure will not capsize. Even ifthe caisson should sink, each cell will resist water pressure so thatthe structure later can be brought to the surface and repaired.

Furthermore, dividing the structure in many small, independent cells hasthe advantage that the ballast cannot shift appreciably. and thereduction in metacenter height due to internal free water surfacebecomes insignificant.

A division into small cells also gives the advantage in that trimming iseasily done. If the sea floor is not level, this can be compensated to acertain degree by more weight on the highest side.

The round form is advantage with respect to breakage of the walls.

To obtain the effects mentioned above, the number of cells must not betoo small. The number should not be below 8.

A particularly preferred form is shown on FIG. 2. As

5 is seen from this figure, all the cells have the same outer diameter.There is one central cell and around this there are first 6 cells withthe centers on a circle concentric with the central cell, thereafter 12cells are placed outside as shown on the figure. If desirable, new rowsof cells can be added on the outside as shown on FIG. 13, where a newrow of I8 cells is added.

The structure can also be constructed without the central cell.

The cells can be closed with a spherical shell in each end as shown onthe figures. Also the ends will thereby be pressure structures. If theedge angle of the spherical shell is too small, tensile stresses willarise at the ends of the cylinder. These must be taken by prestressedreinforcements if cracks in the structure are to be avoided. It has beenfound, however, that if the edge angle of the spherical shell is largerthan approximately 55, no tensile stresses in the cylinder will arise,and the structure will only have compression when it is subjected topure water pressure.

One or more of the cells may be extended up over the water'surface andform the foundation for a working platform. The extention, or the tower,ought to be conical because this will substantially reduse the waveforces without redusing the critical cross section at the bottom.

It can often be advantageous to fit the caisson with a skirt to preventthe sea floor from being washed out from under the structure etc. Thisis easily obtained according to the invention by extending the cylinderwalls downwards. The skirt will thereby have the same good resistance tobreakage as the cell walls while, at the same time, transmission offorces between the skirt and the cell becomes very simple.

By use of ballast, the center of gravity of the caisson can be broughtlow enough for the caisson to be stable per se during towing andsubmerging. Several towers with sufficient distance between them willalso add to the stability.

Small deviations from the even thickness and circular form of the cellsare contemplated, for instance for obtaining a form which can bettersustain shear forces.

If desirable, a caisson according to the invention will be very easy toprestress. Due to the form, however, the water pressure will act asprestressing and prestressing will therefor normally not be necessary.

Prestressing necessary due to tension is conceivable at the followinglocations:

a. the towers in vertical direction due to wave, wind and currentforces;

b. the caisson as a whole if the internal pressure hecomes greater thanthe external pressure which is conceivable if the caisson is used forinstance for oil storage and the oil surface is not kept below the watersurface;

c. the bottom section when the caisson is filled with water due to theweight of the ballast;

d. upper and lower part of cell wall if the spherical shells are tooflat.

By keeping the internal pressure always lower than the externalpressure, all the above mentioned cases of tension can be avoided.

A method for construction a caisson in accordance with the invention isshown schematicly on FIGS. 3-12. As will be seen. the bottom section isfirst made in a dry dock, this section is floated to deep water and thecaisson is finished there. On the figures it is shown that the cylinderwalls are made with a sliding form and that the same is true for thetower. This method makes it very easy to construct the cellsmonolithicly connected which is of great importance for the strength ofthe caisson.

Although but two embodiments of the invention and one method ofconstruction have been described in the preceding and showed on thefigures. it will be apparent to those skilled in the art that variouschanges and modifications may be made therein without departing from thespirit of the invention or the scope of the appended claims.

I claim:

1. A monolithic offshore platform comprising: a heavy sub-structurewhich is heavier than the rest of the platform. the sub-structure beingformed by a plurality of rigidly interconnected. vertical elongatedcells; and comprising six cylindrical cells connected monolithically attheir contact points with centers on a circle with radius equal to theouter diameter of the cells and twelve cylindrical cells of equal sizeplaced outside the six cells and also connected monolithically at theircontact points; a superstructure having a crosssectional area which isexposed to wave action which area is considerably less than thecross-sectional area of the sub-structure; the superstructure beingformed by at least three vertical elongated cells. each of which havingan outer and inner wall at least a portion of which are conical with alarger lower diameter and a smaller upper diameter; and a deck structuresupported by the superstructure; none of said parts of the platformbeing moveable in relation to each other.

2. An offshore platform as claimed in claim 1 including a central cellof equal size.

3. An offshort platform as claimed in claim 1, wherein thesuperstructure is formed by at least three vertically elongated cellssuperposed above three of the cells in the substructure, the lowerdiameter of each of the cells in the superstructure being equal to thediameter of the corresponding cell in the sub-structure.

4. A monolithic off shore platform comprising in combination a heavysubstructure which is heavier than the rest of the platform, thesubstructure being formed by a plurality of rigidly interconnectedvertical elongated cells; a superstructure having a cross-sectional areawhich is exposed to wave action, which area is considerably less thanthe cross-sectional area of the substructure; the superstructure beingformed by at least one vertical elongated cell. each such cell beingformed as a static and continual elongation of one or more cells in thesubstructure and each being conical at least along a portion of itslength with a larger outer and inner lower diameter and a small outerand inner upper diameter; and a deck structure supported by thesuperstructure; none of said parts of the platform being moveable inrelation to each other.

5. A monolithic off shore platform as claimed in claim 4, wherein eachcell in the substructure which is lengthened to form the superstructure,has a circular cross-sectional area.

6. A monolithic off shore platform as claimed in claim 5. wherein thelower diameter of each of the cells in the superstructure issubstantially equal to the diameter of the corresponding cell in thesubstructure.

7. A monolithic off shore platform as claimed in claim 4, wherein theportion of the cells in superstructure which is exposed to water level,has a cylindrical shape.

8. A monolithic off shore platfomt as claimed in claim 1 wherein saidcells are cylindrical.

k l I! l

1. A monolithic offshore platform comprising: a heavy substructure which is heavier than the rest of the platform, the sub-structure being formed by a plurality of rigidly interconnected, vertical elongated cells; and comprising six cylindrical cells connected monolithically at their contact points with centers on a circle with radius equal to the outer diameter of the cells and twelve cylindrical cells of equal size placed outside the six cells and also connected monolithically at their contact points; a superstructure having a cross-sectional area which is exposed to wave action which area is considerably less than the cross-sectional area of the sub-structure; the superstructure being formed by at least three vertical elongated cells, each of which having an outer and inner wall at least a portion of which are conical with a larger lower diameter and a smaller upper diameter; and a deck structure supported by the superstructure; none of said parts of the platform being moveable in relation to each other.
 2. An offshore platform as claimed in claim 1 including a central cell of equal size.
 3. An offshort platform as claimed in claim 1, wherein the superstructure is formed by at least three vertically elongated cells superposed above three of the cells in the substructure, the lower diameter of each of the cells in the superstructure being equal to the diameter of the corresponding cell in the sub-structure.
 4. A monolithic off shore platform comprising in combination a heavy substructure which is heavier than the rest of the platform, the substructure being formed by a plurality of rigidly interconnected vertical elongated cells; a superstructure having a cross-sectional area which is exposed to wave action, which area is considerably less than the cross-sectional area of the substructure; the superstructure being formed by at least one vertical elongated cell, each such cell being formed as a static and continual elongation of one or more cells in the substructure and each being conical at least along a portion of its length with a larger outer and inner lower diameter and a small outer and inner upper diameter; and a deck structure supported by the superstructure; none of said parts of the platform being moveable in relation to each other.
 5. A monolithic off shore platform as claimed in claim 4, wherein each cell in the substructure which is lengthened to form the superstructure, has a circular cross-sectional area.
 6. A monolithic off shore platform as claimed in claim 5, wherein the lower diameter of each of the cells in the superstructure is substantially equal to the diameter of the corresponding cell in the substructure.
 7. A monolithic off shore platform as claimed in claim 4, wherein the portion of the cells in superstructure which is exposed to water level, has a cylindrical shape.
 8. A monolithic off shore platform as claimed in claim 1 wherein said cells are cylindrical. 